Imagine that you can see individual molecules. You watch a small collection of molecules that are moving around slowly while vibrating and bumping against one another. The slower-moving molecules then start to line up, but as they do so their vibrations increase. Soon all the molecules are aligned and vibrating about fixed positions.

The sample is being

(a) cooled and the material is freezing.

(b) heated and the material is melting.

(c) cooled and the material is condensing.

(d) heated and the material is boiling.